1. Field of the Invention
The present invention relates to a semiconductor device using a substrate having flexibility, more specifically, it relates to a semiconductor device using an organic resin substrate.
2. Prior Art
Conventionally known are thin film solar cells using a flexible organic resin film (also called as a plastic film) as a substrate.
Those thin film solar cells using flexible substrates have a larger field of application and can be more easily handled as compared with the solar cells using hard substrates such as glass substrates.
Known flexible substrates include those of polyethylene terephthalate films commonly called by the name of "PET films" and polyimide films. The general use PET films are low cost and readily available, and are characterized by their relatively small linear expansion coefficient of about 3.times.10.sup.5 /.degree.C. Furthermore, because they transmit light, PET films can be used in the same manner as the glass substrates for fabricating thin solar cells of a type in which the incident light is introduced from the substrate side.
A thin film solar cell can be fabricated by a known process using a PET film in the same manner as a one using a glass substrate. That is, an amorphous silicon (denoted generally as "a-Si") semiconductor is deposited on the substrate by a chemical vapor reaction process. In such a process, the temperature of the substrate must be maintained low because a PET film, as well as an organic resin film, has poor thermal resistance.
As described in the foregoing, care had been taken in using an organic resin film such as a PET film in a known process for fabricating thin film solar cells so that the substrate temperature may not become excessively high, because heat is fatal to a plastic film. However, when a transparent conductive film such as an ITO (Indium-Tin-Oxide) is deposited by sputtering on a substrate made of, e.g., a PET film, the surface of the film becomes hot due to the sputtered ITO particles that hit the surface even though the film is not externally heated. As a result, the unreacted starting materials, fillers, UV absorbers, etc., which had been included in the PET film precipitate as oligomers to form irregularities on the surface of the film. An oligomer is a polymer having a lower polymerization degree, and has a melting point or a softening point lower than that of polyethylene terephthalate which constitutes the PET film.
Furthermore, the substrate should be heated on depositing an amorphous silicon semiconductor by plasma CVD (chemical vapor deposition) to establish the photoelectric conversion layer. This step somewhat enhances growth of the oligomer having precipitated on the film by depositing the ITO film above, thereby leaving film irregularities being grown to a size of about 1 .mu.m.
As is well known, the amorphous silicon semiconductor layer (generally having a PIN structure) which corresponds to the photoelectric conversion layer is deposited at a thickness of from about 0.2 to 1 .mu.m. It can be seen therefore that the amorphous silicon film is greatly influenced by the irregularities which generate with the precipitation of the oligomers. This severely impairs the conversion efficiency because the thickness of the photoelectric conversion layer remains no longer constant.
Referring to an example as shown in FIG. 2, the effect of the precipitated oligomer on the structure of a thin film solar cell is explained. The figure shows schematically a cross sectional view of a solar cell, which was constructed based on a micrograph obtained by SEM (scanning electron microscopy) and an analysis using SIMS (secondary ion mass spectroscopy).
In FIG. 2, there is shown a structure comprising a 100 .mu.m thick organic resin film substrate, i.e., a PET film substrate 20, having deposited thereon a 4,000 .ANG. thick ITO electrode 22 and a 4,500 .ANG. thick photoelectric conversion layer 23 composed of an amorphous silicon having a PIN structure in this order from the substrate side. An aluminum back electrode 24 is established at a thickness of 3,000 .ANG.. Spherical oligomers 21 about 1 .mu.m in diameter are incorporated between the PET film 20 and the photoelectric conversion layer 23. It can be seen clearly from the figure that the oligomers as large as 1 .mu.m in diameter greatly affect both the pair of electrodes and the photoelectric conversion layer. The presence of such oligomers causes the thickness of the photoelectric conversion layer 23 to fluctuate, and it thereby lowers the conversion efficiency. Furthermore, because the oligomers grow to penetrate the first electrode on the substrate, the incorporation of a plurality of oligomers causes an increase in resistivity of the first electrically conductive film. This is also another problem to be solved. Moreover, it tends to become complicated on taking an integrated structure by connecting and separating the electrodes.
As described above, the thin film solar cell which is obtained on an organic resin substrate suffered lower conversion efficiency as compared with a photoelectric conversion device established on a glass substrate, because in the former, the presence of the oligomers which precipitate on depositing the transparent electrode and which develop during the deposition of an amorphous silicon layer considerably impaired the efficiency.
In general, the film deposition temperature is of great concern in depositing a thin amorphous silicon film by plasma CVD on an organic resin substrate. It is known that a substrate temperature in the range of from about 100.degree. to 300.degree. C. is optimal in depositing amorphous silicon on a glass substrate. Accordingly, a substrate temperature of 100.degree. C. or higher is assumably favorable for depositing a thin amorphous silicon film on a PET film. However, the thermal expansion of the PET film then arises as another problem in depositing the film at high temperatures.
More specifically, an amorphous silicon film tends to fall off, or cracks generate on the substrate when the substrate undergoes thermal expansion. This problem occurs not only on PET films, but also on all organic resin films which are used as substrates in fabricating semiconductor devices. The problem above can be well understood by considering, as mentioned hereinbefore, the fact that a PET film has a relatively small thermal expansion coefficient among the organic resin films.
Furthermore, even when the problem of thermal expansion is overcome by some means, the problem of growing oligomers is yet to solve; the oligomers having precipitated with the film deposition of the transparent conductive films grow during the deposition of the thin film amorphous silicon.
As mentioned in the foregoing, a conventional process of depositing amorphous silicon on a PET film by plasma CVD process suffered two problems, i.e., one is the problem of the oligomers which inevitably precipitate from the PET film, and the other is the problem of the thermal expansion of the PET film. Consequently, the film deposition temperature had been determined optimally by taking the both problems into account.
Conventional organic resin substrates in general contain fillers. The fillers function as a reinforcing material or enable the organic resin substrate to easily pass through the rollers. The addition of such fillers, however, forms irregularities on the surface of the resin substrate. Accordingly, characteristics of a photoelectric conversion device and the like which is established on such a substrate tends to be impaired by the surface irregularities of the substrate.